Documents the OpenAI-compatible endpoint update across the root README files.\n\nChanges:\n- Adds the OpenAI-compatible local backend section to README.ko.md, README.zh.md, and README.ja.md.\n- Keeps README.md aligned with the compatibility audit by mentioning Python/JS SDK smoke coverage and remaining full-stack gates.\n- Links every language README to docs/openai-compatibility.md and docs/openai-compatibility-audit.md.\n\nVerification:\n- git diff --check\n- Verified linked docs files exist.